The present invention relates generally to test and diagnostic equipment for testing motor vehicles, particularly vehicles powered by internal combustion engines. The invention has particular application to diagnostic equipment incorporating gas analyzers for analyzing exhaust emissions from internal combustion engines, and even more specifically to nitric oxide (NO) sensor circuits for such gas analyzers.
The present invention is an improvement of a NO sensing circuit of the type used with a diagnostic system, such as that sold by Sun Electric and known as a Service Inspection System. That system includes an infrared (IR) shelf assembly module, which includes a non-dispersive infrared (NDIR) optical bench which detects the concentration of hydrocarbons, carbon monoxide, carbon dioxide and other gases within the vehicle exhaust system. The NDIR optical bench includes optional input/output circuits and peripheral transducers for additional inputs, including a NO input.
There are government regulations setting forth specifications for the performance of engine diagnostic equipment and, in particular, emissions analyzers. Among these specifications is a response time specification for certain gas constituent sensors. The specifications essentially require that the sensor output reach a certain percentage of a nominal output reading within a certain time period, e.g., within four or five seconds, the specified time period varying with the ambient temperature at which the test is conducted. Applicants have found that when the NO sensor was utilized in an emissions analyzer, its response times, i.e., the rise and fall times of the sensor output, could exceed the specifications set by government regulations, particularly at low ambient temperatures. Applicants attempted heating the NO sensing cell, as with a resistive heater, but the heater did not decrease response times enough to meet specifications.